SPIDER Cs Ovens functional tests

Abstract The SPIDER experiment is operating at the ITER Neutral Beam Test Facility PRIMA since June 2018 to study the operation of a full ITER-size negative ion source. In order to increase the negative ion extracted current and reduce the co-extracted electron current, caesium is evaporated inside the source. In fact, the adsorption of Cs onto the plasma grid, i.e. the first grid of the extraction system, lowers the surface work function, enhancing the probability of surface conversion of impinging hydrogen particles into negative ions. At the SPIDER test facility, Cs will be evaporated by heating a liquid Cs reservoir in three Cs ovens located at the back-plate of the ion source. The ovens must assure a controllable and reproducible evaporation. A SPIDER Cs oven prototype was procured and tested in the CAesium Test Stand (CATS) at PRIMA site to validate the design. Following the experimental tests, few improvements were introduced in the final design of the three Cs ovens for the SPIDER source. Then, the ovens were procured in 2019 and commissioned before their installation in SPIDER in 2020. This paper reports the functional tests of the SPIDER Cs ovens and of their embedded diagnostics. These tests were carried out at CATS, with the aim of characterizing the three ovens, in order to validate them for their use in SPIDER in the experimental campaign with caesium evaporation planned for the end of 2020. The ovens were successfully tested and characterized in terms of evaporation rate for different oven temperatures. Moreover, the three ovens show a good reproducibility and reliability.